Optical fiber-based three-dimensional imaging system

ABSTRACT

Described are an imaging device and method for determining three-dimensional position information of a surface of an object. The device includes a pair of optical fibers, a phase shifter, a detector array and a processor. The phase shifter is coupled to one of the optical fibers and is used to change a phase of optical radiation emitted from the optical fiber relative to a phase of optical radiation emitted from the other optical fiber. The detector array receives optical radiation scattered by the surface of the object. The processor communicates with the detector array and the phase shifter. Signals generated by the detector array are received by the processor and three-dimensional position information for the surface is calculated in response to the received optical radiation scattered by the surface of the object and the change in the relative phase of optical radiation emitted by the optical fibers.

RELATED APPLICATIONS

This application claims the benefit of the earlier filing date of U.S.Provisional Patent Application Ser. No. 60/984,452, filed Nov. 1, 2007,titled “High Accuracy Three-Dimensional Imaging of Polished andTranslucent Material,” and U.S. Provisional Patent Application Ser. No.60/984,467, filed Nov. 1, 2007, titled “Fiber-Based Accordion FringeInterferometry,” the entireties of which are incorporated herein byreference.

FIELD OF THE INVENTION

The invention relates generally to the measurement of surface contoursand more particularly to a non-contact apparatus using opticalfiber-based accordion fringe interferometry (AFI) for thethree-dimensional measurement of objects.

BACKGROUND OF THE INVENTION

The process for determining the shape of teeth according to traditionaldentistry generally includes the use of impression materials, molds orcastings. This process is typically slow and prone to material handlingerrors. After obtaining the impression of the patient's teeth, the moldor impression material is removed from the mouth of the patient and asolid model of the patient's teeth is made from the impression. Theimpression material or the solid model is sent to a dental laboratory.The solid model is used in the fabrication of one or more corrective orreplacement dental components such as artificial teeth, crowns ororthodontic appliances. Inaccuracies and errors introduced at any timeduring the process can result in an improper fit of the dental componentand may limit the ability to secure and retain the dental component inthe correct location.

U.S. Pat. No. 4,964,770 describes a process for making artificial teeth.The process includes projecting contour lines onto the patient's teethand detecting the contour lines using a camera. The location of theprojected contour lines is shifted multiple times by a precision motionof the projector and detected by the camera at each position. Cameradata are processed to determine contour data for the teeth. The contourdata may be provided to a numerically controlled fabrication machine forthe generation of the artificial teeth or for orthodontic appliances orfor use with dental implantology. The process is subject to inaccuraciesas displacement of the contour lines is based on changing the locationof the projector. Moreover, the length of time required to obtain thecamera data for all sets of contour lines is a significant inconvenienceto the dental patient and makes the measurement more sensitive to motionof the projection source, teeth and camera.

Material characteristics of teeth can further limit the ability toobtain accurate three-dimensional data. Teeth are typically translucenttherefore a portion of the light incident on the surface of a tooth isscattered from the surface while some of the light penetrates thesurface and is internally scattered over a depth below the surface.Furthermore, backscatter can occur at the interface of tooth enamel anddentin if there is sufficient penetration of the incident light.Translucency can prevent an accurate determination of the surfacecontour of teeth using optical techniques. For example, projectedcontour lines may appear shifted from their actual location and may havepoor contrast. In some instances, translucency causes measurements basedon optical techniques to indicate an apparent surface that is beneaththe true surface. To overcome difficulties due to translucency, dentistsoften apply powders such as titanium dioxide to teeth. The applicationof powder is a further inconvenience that adds more time to themeasurement process, introduces measurement uncertainty and caninterfere with adhesives or other bonding agents used to fasten thereplacement tooth.

SUMMARY OF THE INVENTION

In one aspect, the invention features a method for determiningthree-dimensional position information of a surface of an object. Thesurface of the object is illuminated with radiation emitted from a pairof optical fibers. The radiation emitted from each optical fiber iscoherent with respect to the radiation emitted from the other opticalfiber. Radiation scattered from the surface of the object is detected. Aphase of the radiation emitted from one of the optical fibers relativeto a phase of the radiation emitted from the other optical fiber asdetermined at a point on the surface of the object is changed beforeagain detecting radiation scattered by the surface of the object.Three-dimensional position information of the surface of the object iscalculated in response to the change of the phase and the radiationdetected before and after the change of the phase.

In another aspect, the invention features an imaging device fordetermining three-dimensional position information of a surface of anobject. The device includes first and second optical fibers, a phaseshifter, a detector array and a processor. Each optical fiber has anexit end and is adapted to receive optical radiation. The opticalradiation received by the first and second optical fibers is mutuallycoherent. The phase shifter is coupled to the first optical fiber tochange a phase of optical radiation emitted from the exit end of thefirst optical fiber relative to a phase of optical radiation emittedfrom the exit end of the second optical fiber. The detector arrayreceives optical radiation scattered by the surface of the object. Theprocessor communicates with the detector array and the phase shifter,and receives signals generated by the detector array. The processorcalculates three-dimensional position information in response to thereceived optical radiation scattered by the surface of the object and achange in the phase of optical radiation emitted from the exit end ofthe first optical fiber relative to a phase of optical radiation emittedfrom the exit end of the second optical fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of this invention may be betterunderstood by referring to the following description in conjunction withthe accompanying drawings, in which like numerals indicate likestructural elements and features in the various figures. The drawingsare not necessarily to scale, emphasis instead being placed uponillustrating the principles of the invention.

FIG. 1 illustrates an embodiment of an imaging device for determiningthree-dimensional position information of a surface of an objectaccording to the invention

FIG. 2 is a flowchart representation of an embodiment of a method fordetermining three-dimensional position information of a surface of anobject according to the invention.

FIG. 3 is a vector illustration of radiation from the two single modeoptical fibers of FIG. 1 as incident on the surface of a translucentobject.

FIG. 4A illustrates the illumination of a flat surface by a pair ofoptical fibers.

FIG. 4B illustrates the fringe pattern at the flat surface of FIG. 4A.

FIG. 4C illustrates the vertical intensity profile of a portion of thefringe pattern depicted in FIG. 4B.

FIG. 5A shows the displacement of an optical fiber to change therelative phase of the radiation emitted from the two optical fibers ofFIG. 4A.

FIG. 5B illustrates a lateral shift of the fringe pattern of FIG. 4B dueto a change in the relative phase of the radiation emitted from the twooptical fibers according to FIG. 5A.

FIG. 5C illustrates the vertical intensity profile for a portion of thefringe pattern depicted in FIG. 5B.

FIG. 5D illustrates a fiber stretching technique for changing therelative phase of the radiation emitted from the two optical fibers ofFIG. 4A.

FIG. 6A shows a change in the separation of the fiber ends of the twooptical fibers of FIG. 4A used to modify the spatial frequency of afringe pattern.

FIG. 6B illustrates the fringe pattern resulting from the configurationof optical fibers shown in FIG. 6A.

FIG. 6C illustrates vertical intensity profiles for a portion of thefringe patterns depicted in FIG. 4B and FIG. 6B.

FIG. 7 illustrates a structure for use in an embodiment of an intra-oraldevice according to the invention.

FIG. 8 illustrates a configuration of two structures for use in anembodiment of an intra-oral device according to the invention.

DETAILED DESCRIPTION

In brief overview, the present invention relates to a device based onaccordion fringe interferometry (AFI) principles that are useful forreal-time three-dimensional imaging of objects. The device can be usedin a wide variety of applications including, for example, intra-oralimaging for restorative dentistry and orthodontics, handheldthree-dimensional scanners and probes for industrial applications suchas measurement and inspection, and compact three-dimensional machinevision sensors. Compact and substantially insensitive to motion betweenthe device and the objects to be measured, the device is advantageouslyadapted for scanning translucent objects and intra-oral surfaces such asthe surfaces of teeth, gum tissue and various dental structures andmaterials.

The device can be fabricated from inexpensive components used in thehigh volume consumer electronics and telecommunications industries. Inone embodiment, portions of the device are packaged as a small wand thatis easily held and maneuvered by a dental professional. A remoteelectrical power supply and optical source enable a more compact wand.Although the device utilizes AFI measurement techniques as described inU.S. Pat. No. 5,870,191 to Shirley et al., the device does not use agrating and lens to generate coherent point sources of radiation as inother AFI configurations. Instead, radiation is emitted from a pair ofoptical fibers and is used to illuminate objects to be measured withinterferometric fringes. Consequently, movement of a macroscopic gratingwhich requires several milliseconds or more to effect a phase shift isunnecessary. A fiber-based phase shifter is used to change the relativephase (i.e., the difference in phase) of the radiation emitted from theexit ends of the two optical fibers in a few microseconds or less.Optical radiation scattered from surfaces and subsurface regions ofilluminated objects is received by a detector array. Electrical signalsare generated by the detector array in response to the receivedradiation. A processor receives the electrical signals and calculatesthree-dimensional position information of object surfaces based onchanges in the relative phase of the emitted optical radiation and thereceived optical radiation scattered by the surfaces. The devicepreferably utilizes a source of optical radiation having a wavelengthbetween about 350 nm and 500 nm to reduce measurement error associatedwith penetration of the incident radiation into the subsurface regionsof translucent objects.

FIG. 1 illustrates an embodiment of an imaging device for determiningthree-dimensional position information of a surface of an object 10according to the invention. FIG. 2 is a flowchart representation of anembodiment of a method 100 for determining three-dimensional positioninformation of a surface of an object (e.g., object 10) according to theinvention. In the illustrated embodiment, the object 10 is a translucentobject such as an intra-oral object (i.e., an object located in themouth of a patent). For example, the object 10 may be a natural orartificial tooth. Optical radiation is launched into a pair of singlemode optical fibers 14A and 14B from a master optical source (notshown). In one embodiment, the optical fibers 14 are coupled by a fibersplitter so that optical radiation is launched into an input end of asingle optical fiber. The object 10 is illuminated (step 110) with thediverging radiation emitted from the exit ends of the fibers 14. In oneembodiment, the radiation emitted from the two fibers 14 is polarized ina common orientation.

Some of the radiation incident on the translucent object 140 isscattered from the surface while some of the radiation penetrates into asubsurface region (i.e., a volume below the surface) where it isscattered. An image of the surface of the object 10 is formed by animaging element or lens 18 on an array of photodetectors 22 such as atwo-dimensional charge coupled device (CCD) imaging array. The detectorarray 22 provides an output signal to a processor 26. The output signalincludes information on the intensity of the radiation received (steps120 and 140) at each photodetector in the array 22. An optionalpolarizer 30 is oriented to coincide with the main polarizationcomponent of the scattered radiation. A control module 34 controls theoperation of the radiation emitted from the optical fibers 14. Thecontrol module 34 includes a phase shifter 36 that adjusts (step 130)the relative phase of the radiation emitted by the two fibers 14 asdetermined at the surface of the object 10. The control module 34 alsoincludes a spatial frequency controller 38 that adjust the pitch ofinterference fringes 32 in the illumination pattern at the objectsurface. The fringes 32 are the result of interference of the coherentradiation emitted from the optical fibers 14. The spatial frequency ofthe fringe pattern, i.e., the inverse of the separation of the fringes32, is determined by the separation D of the ends of the optical fibers14, the distance from the ends of the fibers to the object, and thewavelength of the radiation. The processor 26 and control module 34communicate to coordinate the processing of signals from thephotodetector array 22 with respect to changes to the spatial frequencyand the relative phase, and the processor 26 determines (step 150) thethree-dimensional information for the surface according to the detectedradiation. In a preferred embodiment, the processor 26 includes multiplecentral processing units (“CPUs”) for parallel processing to increasethe data processing rate and to output the final measurement data inless time.

FIG. 3 is a vector illustration of the radiation 40 from the two singlemode optical fibers as incident on the surface 42 of the translucentobject 10. A portion of the incident radiation 40 is scattered from thesurface 42 while a portion of the incident radiation 40 penetrates andis scattered from within the object 10 below the surface 42. Thewavelength of the incident radiation, and the wavelength-dependentreflectance characteristic and wavelength-dependent transmittancecharacteristic of the object material, determine the relativecontributions to the radiation scattered from the object surface 42 andthe radiation scattered within the subsurface region.

Referring also to FIG. 1, the photodetector array 22 receives an imageof the fringe pattern projected onto the surface 42 of the object 10;however, the radiation scattered in the subsurface region degrades theimage of the fringe pattern. If the scattered radiation contributionfrom the subsurface region is significant relative to the scatteredradiation contribution from the surface 42, the apparent location (i.e.,apparent phase) of the fringe pattern on the surface 42 can be differentthan the actual location. The method of the invention exploits acounter-intuitive approach to reducing measurement errors due totranslucent internal scatter by utilizing an illumination wavelengthwhich increases internal scatter near the surface. The illuminationwavelength exhibiting a high coefficient of internal scattering iscombined with a high spatial frequency fringe pattern such that a nearlyuniform photon flux is created just below the surface. The subsurfacescatter contributes a substantially constant background to the image ofthe surface 42 at the photodetector array 22. The backgroundcontribution is ignored by the phase analysis, and the magnitude of theresidual error induced by any remaining spatially-varying intensitycontribution is further reduced in significance if the contribution isclose to the surface 42. Measurements performed according to the methodof the invention provide greater accuracy when using radiation having awavelength predetermined from the wavelength-dependent reflectancecharacteristic and wavelength-dependent transmittance characteristic ofthe object material to improve the surface scattering contributionrelative to the subsurface scattering contribution.

For three-dimensional imaging of living, natural teeth, wavelengths inthe lower visible and near ultraviolet (UV) range (e.g., 350 to 500 nm)provide a higher coefficient of subsurface scattering than longerwavelengths due to the wavelength-dependent characteristics of enameland dentin. In one embodiment of an intra-oral imaging device, theradiation source is a commercially-available blue laser diode having anoperating wavelength of 405 nm (e.g., model no. BCL-050-405 availablefrom Crystal Laser of Reno, Nev.).

In another embodiment of an intra-oral imaging device, the spatialfrequency of the fringe pattern at the surface 42 is predetermined sothat subsurface scattering of radiation that penetrates the object 10from one fringe overlaps the subsurface scatter from the two neighboringfringes, and thus diffuses the fringes 32 that propagate below thesurface. As a result, the scattered subsurface radiation provides asubstantially constant background to the detected fringe pattern. In apreferred embodiment, the spatial frequency of the fringe pattern is atleast 1 fringe/mm.

FIG. 4A illustrates how radiation emitted from a pair of single modeoptical fibers 14 illuminates a flat surface 46 resulting in the fringepattern shown in FIG. 4B where a dark horizontal line indicates thegreatest intensity for each fringe. The fringes have a sinusoidalintensity profile as shown in FIG. 4C for a vertical slice through theboxed region 50 of FIG. 4B. To rapidly change the relative phase of theradiation emitted from the two fibers 14, the phase shifter 36 includesa translation module to move the end of one of the fibers 14B along thefiber axis (i.e., z-axis) to displace the end of the fiber 14B from itsoriginal position by a distance Δ as shown in FIG. 5A. Thus if the fiberend is moved through a distance that is one-third of the wavelength λ(i.e., Δ=λ/3) of the radiation, the relative phase shift is 120°. FIGS.5B and 5C illustrate the shift in the fringe pattern that results from a120° phase shift. In one embodiment, the mechanism used to translate thefiber end is a translation stage that can accurately position the end ofthe fiber 14B along the z-axis in either direction.

FIG. 5D illustrates a different technique for obtaining a relative phaseshift. In this technique, the phase shifter 36 includes a fiber stretchmechanism to stretch one of the optical fibers 14B along a length Lwhile its end remains fixed in location. The stretching alters thephysical properties of the fiber 14B such as its length, index ofrefraction, and birefringence. As a result, the effective optical pathlength of the fiber 14B is changed and the phase of the radiationemitted from the fiber 14B is changed relative to the phase of theradiation emitted from the other fiber 14A. This change in the relativephase results in a lateral shift of the fringe pattern as shown, forexample, in FIG. 5B and FIG. 5C. In an alternative embodiment, the phaseshifter 36 includes a fiber compression mechanism to compress or “pinch”one of the optical fibers 14 along a radial fiber axis (i.e., an axis inthe x-y plane).

For a constant wavelength, the spatial frequency of the fringe patternis adjusted by controlling the separation D of the fiber ends. FIG. 6Ashows how the separation D of the fiber ends is decreased relative tothe fibers 14 of FIG. 4A. As a result, the fringe pattern and thefringes within the fringe pattern are broadened as shown in FIG. 6B andFIG. 6C. In one embodiment, the spatial frequency controller 38 (FIG. 1)includes a translation module to translate the end of at least one ofthe optical fibers 14 along a transverse axis defined between the fiberends (i.e., the illustrated x-axis) in either direction. The separationdistance D is decreased to reduce the spatial frequency or increased toincrease the spatial frequency. Preferably, both fiber ends aretranslated equal distances in opposite directions so that the positionof the midpoint between the fiber ends is unchanged and the fringepattern is not laterally shifted.

FIG. 7 illustrates a structure 62 having axial and lateral flexures foruse in an embodiment of an intra-oral device according to the invention.Two single mode optical fibers 14 are fixed by clamps 66 to a substrate70. The substrate 70 can be fabricated from aluminum or another materialdepending on the tolerance to temperature variations and fabricationcapabilities. Two piezoelectric actuators 74A and 74B are secured to thesubstrate 70. The actuators 74 provide a means to change the separationdistance between the exit ends of the optical fibers 14 and to changethe axial position of the exit end of one fiber 14B. Initially, acontrol signal is applied to each actuator 74 to adjust its positionapproximately to a midrange position within an operating range (e.g., ±1μm range). Optical fiber 14A is then clamped to the lateral flexureportion of the substrate 70. Next, optical fiber 14B is positioned andadjusted along its fiber axis to maximize the contrast of the fringepattern, and then clamped to the axial flexure portion of the substrate70. During measurements, the axial piezoelectric actuator 74A adjuststhe axial position of the end of the optical fiber 14B over a portion ofthe actuator range to cause a shift in the relative phase by the desiredamount (e.g., +120° and −120°). The lateral piezoelectric actuator 74Badjusts the separation of the ends of the two fibers 14 within a rangeof ±1 μm to cause a change in the spatial frequency of the fringepattern by a desired value.

FIG. 8 illustrates a device configuration employing two structures 78and 82 for use in an embodiment of an intra-oral device according to theinvention. The two single mode optical fibers 14 are fixed by clamps 66to structures 78 and 82. One piezoelectric actuator 74B is integratedinto one structure 82 to enable a controlled change in the separationdistance between the ends of the optical fibers 14. The otherpiezoelectric actuator 74A is used to stretch an optical fiber 14A (asdescribed for FIG. 4D) and thereby change the relative phase between theradiation emitted from the ends of the two fibers 14. During initialconfiguration setup, control signals position the actuators 74 near themidpoints of their respective ranges of motion. Optical fiber 14B isthen clamped to the structure 78 near the center of the lateral flexureportion and near the exit end. Optical fiber 14A is then clamped to thestructure 78 near the center of the lateral flexure and near theentrance end. The axial piezoelectric actuator 74A is used to stretchone optical fiber 14A on expansion and to stretch the other opticalfiber 14B on contraction. Stretching of the optical fibers 14 in thismanner causes a shift in the relative phase of the radiation emittedfrom the ends of the fibers 14 by the desired amount (e.g., +120° and−120°). As the fiber clamps 66 are in contact with the sheathing of eachfiber 14, the linear motion of the actuator 74A generally issubstantially greater than the actual increase in length of thestretched optical fiber 14. Furthermore, the optical path length of theoptical radiation is dependent on the length and the index of refractionof each optical fiber 14. Consequently, the desired relative phase shiftis preferably determined by calibrating an initial position for eachphase, and then by dynamically tracking fringe motion and applying acompensation offset.

In an alternative embodiment, a second separation of the optical fibers14 is effectively accomplished by selectively enabling a third opticalfiber (not shown) spaced the proper distance from the first fiber 14B.Thus the translation mechanism can be eliminated.

After completing attachment to the first structure 78, the opticalfibers 14 are attached to the second structure 82 that includes alateral piezoelectric actuator 74B for achieving a desired separation ofthe fiber ends. In one embodiment, the first and second structures 78and 82 are separated by at least 50 mm to provide sufficient slack inthe two optical fibers 14 so that motion of one actuator 74 does notaffect the control of the fibers 14 by the other actuator 74.

Environmental instability can limit the ability to perform accuratemeasurements using AFI principles. In particular, measurement accuracyis degraded according to the error between the commanded phase shift andthe actual phase shift applied to the fringe pattern. The phase shiftingtechniques described above impart a small optical path length differenceof a few hundred nanometers or less between the two radiation beams.Consequently, temperature drift and mechanical creep can be sources oferror in the relative phase. Capacitance gauges can be used to monitoraxial shifts in optical fiber position and stretching or compressionactuation for the phase shifting techniques described above; however, insome embodiments such electro-mechanical monitoring may not accuratelycorrespond to changes imparted to the relative phase.

In a preferred embodiment, a control system based on fringe monitoringis used to establish the desired relative phase and to maintain fringestability. Referring again to FIG. 1, a portion of the fringe pattern is“sampled” by an optical element 54 and received by a fringe positionsensor 58 that determines the location of a fringe within a small regionof the fringe pattern. In one embodiment, the fringe position sensor 58is a detector array. In other embodiments the fringe position sensorincludes a configuration of one or more detectors that can sense achange in the position of a fringe. In another embodiment, the fringepattern detector may be a section of the detector array 22. The fringeposition detector array 58 can have a low pixel count as only one fringein the fringe pattern is monitored. The optical element 54 can be amirror having a dimension that is approximately equal to the width of afringe at the mirror. The reflected radiation is directed to thedetector array 58 which provides image data to the processor 26. Theimage data are processed to monitor the location of the sampled fringein the field of view of the detector array 58. If the processor 26determines that the fringe is “drifting” and therefore that there is adisplacement error in the fringe pattern, the phase shifter 36 adjuststhe relative phase to counteract the drift. Thus the displacement erroris substantially cancelled and the fringe pattern remains stable.

While the invention has been shown and described with reference tospecific embodiments, it should be understood by those skilled in theart that various changes in form and detail may be made therein withoutdeparting from the spirit and scope of the invention.

1. A method for determining three-dimensional position information of asurface of an object, the method comprising: illuminating the surface ofthe object with radiation emitted from a pair of optical fibers, theradiation emitted from each optical fiber being coherent with respect tothe radiation emitted from the other optical fiber; detecting radiationscattered from the surface of the object; changing a phase of theradiation emitted from one of the optical fibers relative to a phase ofthe radiation emitted from the other optical fiber as determined at apoint on the surface of the object; detecting radiation scattered by thesurface of the object after the change of the phase; and calculatingthree-dimensional position information of the surface of the object inresponse to the change of the phase and the radiation detected beforeand after the change of the phase.
 2. The method of claim 1 whereinchanging the phase of the radiation emitted from one of the opticalfibers relative to the phase of the radiation emitted from the otheroptical fiber comprises translating an end of one of the optical fibersalong a fiber axis.
 3. The method of claim 1 wherein changing the phaseof the radiation emitted from one of the optical fibers relative to thephase of the radiation emitted from the other of the optical fiberscomprises stretching one of the optical fibers along a fiber axis. 4.The method of claim 1 wherein changing the phase of the radiationemitted from one of the optical fibers relative to the phase of theradiation emitted from the other of the optical fibers comprisescompressing one of the optical fibers along a radial fiber axis.
 5. Themethod of claim 1 wherein illuminating the surface of the objectcomprises generating a fringe pattern on the surface of the object. 6.The method of claim 5 further comprising changing a spatial frequency ofthe fringe pattern.
 7. The method of claim 6 wherein each optical fiberhas an end separated from an end of the other optical fiber along atransverse axis and wherein changing the spatial frequency of the fringepattern comprises translating the end of at least one of the opticalfibers along the transverse axis.
 8. The method of claim 5 furthercomprising: determining a displacement error in the fringe pattern; andchanging the phase of the radiation emitted from one of the opticalfibers relative to the phase of the radiation emitted from the otheroptical fiber to substantially cancel the displacement error.
 9. Animaging device for determining three-dimensional position information ofa surface of an object, the imaging device comprising: a first opticalfiber adapted to receive optical radiation and having an exit end; asecond optical fiber adapted to receive optical radiation and having anexit end, the optical radiation received by the first and second opticalfibers being mutually coherent; a phase shifter coupled to the firstoptical fiber to change a phase of optical radiation emitted from theexit end of the first optical fiber relative to a phase of opticalradiation emitted from the exit end of the second optical fiber; adetector array to receive optical radiation scattered by the surface ofthe object; and a processor in communication with the detector array andthe phase shifter, the processor receiving signals generated by thedetector array and calculating three-dimensional position information inresponse to the received optical radiation scattered by the surface ofthe object and a change in the phase of optical radiation emitted fromthe exit end of the first optical fiber relative to a phase of opticalradiation emitted from the exit end of the second optical fiber.
 10. Theimaging device of claim 9 wherein the phase shifter comprises atranslation module coupled to the first optical fiber and adapted forchanging a position of the exit end of the first optical fiber along afiber axis.
 11. The imaging device of claim 9 wherein the phase shiftercomprises a fiber stretch mechanism coupled to the first optical fiberand adapted for stretching a length of the first optical fiber.
 12. Theimaging device of claim 9 wherein the phase shifter comprises a fibercompression mechanism coupled to the first optical fiber and adapted forcompressing the first optical fiber along a radial fiber axis.
 13. Theimaging device of claim 9 wherein the optical radiation incident on thesurface of the object comprises a fringe pattern and wherein the devicefurther comprises a translation module coupled to one of the opticalfibers to change a separation of the exit ends of the optical fibers andthereby to change a spatial frequency of the fringe pattern.
 14. Theimaging device of claim 9 further comprising a source of opticalradiation in communication with the first and second optical fibers. 15.The imaging device of claim 9 wherein the optical radiation incident onthe surface of the object comprises a fringe pattern, the device furthercomprising: a fringe position sensor in communication with theprocessor; and an optical element disposed between the object and theexit ends of the first and second optical fibers, the optical elementconfigured to direct a portion of the fringe pattern to the fringeposition sensor, wherein the processor adjusts the phase of opticalradiation emitted from the exit end of one of the optical fibersrelative to the phase of optical radiation emitted from the exit end ofthe other optical fiber in response to a signal from the fringe positionsensor indicating a phase error in the fringe pattern.